Grease Pencil-Preceding frame visible in next frame(non onion skin)-turn off please
This is defacto standard in 2D-animation. This behavior makes easy to draw on twos, threes, ets. without adding additional keyframes. (When you draw every second frame it is called "drawing on twos", every third - "drawing on threes" and so on.) Timeline looks clear and ones, twos and threes easy to recognize and manipulate with them. 

Know more why using twos is important in 9th Principle of Animation